U.S. patent application number 13/673324 was filed with the patent office on 2014-05-15 for microphone system with mechanically-coupled diaphragms.
This patent application is currently assigned to Invensense, Inc.. The applicant listed for this patent is Invensense, Inc.. Invention is credited to Fang Liu, Kuang L. Yang.
Application Number | 20140133685 13/673324 |
Document ID | / |
Family ID | 50681715 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140133685 |
Kind Code |
A1 |
Liu; Fang ; et al. |
May 15, 2014 |
Microphone System with Mechanically-Coupled Diaphragms
Abstract
A microphone system has two diaphragms and are mechanically
interconnected such that they respond in antiphase to an acoustic
signal impinging on one of the diaphragms. The two diaphragms
produce two variable capacitances that vary proportionately but
inversely to one another. Voltage signals produced by the two
variable capacitances are summed to provide an output signal
proportional to the acoustic signal, but with greater sensitivity
than a single-diaphragm microphone.
Inventors: |
Liu; Fang; (Woburn, MA)
; Yang; Kuang L.; (Newton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Invensense, Inc. |
San Jose |
CA |
US |
|
|
Assignee: |
Invensense, Inc.
San Jose
CA
|
Family ID: |
50681715 |
Appl. No.: |
13/673324 |
Filed: |
November 9, 2012 |
Current U.S.
Class: |
381/355 ;
381/174 |
Current CPC
Class: |
H04R 19/005 20130101;
B81B 3/0056 20130101; H04R 19/04 20130101; B81B 2201/0221 20130101;
H04R 7/20 20130101; H04R 3/005 20130101; H04R 31/006 20130101; H04R
7/06 20130101; B81B 2201/0257 20130101; H04R 1/2807 20130101 |
Class at
Publication: |
381/355 ;
381/174 |
International
Class: |
H04R 1/04 20060101
H04R001/04 |
Claims
1. A microphone system for detecting an acoustic signal, the
microphone system comprising: a micromachined device comprising a
backplate; a primary diaphragm separated from the backplate by a
variable primary gap, the primary diaphragm and the backplate
forming a variable primary capacitance across the primary gap, the
primary capacitance varying in response to the acoustic signal
impinging on the primary diaphragm; a reference electrode; a
reference diaphragm separated from the reference electrode by a
variable reference gap, the reference diaphragm forming a variable
reference capacitance with the reference electrode across the
variable reference gap; and a mechanical coupler coupling the
primary diaphragm to the reference, diaphragm, the mechanical
coupler being configured to vary the reference gap inversely and
proportionately to the primary gap, the mechanical coupler also
being configured to vary the reference capacitance inversely and
proportionately to the reference capacitance.
2. The microphone system of claim 1, wherein the mechanical coupler
comprises: a torsion bar supported by at least one anchor coupled
to a substrate, the torsion bar between the primary diaphragm and
the reference diaphragm; and a beam coupled to the torsion bar, the
primary diaphragm, and the reference diaphragm, the beam
mechanically coupling the primary diaphragm to the reference
diaphragm.
3. The microphone system of claim 1, wherein the primary
capacitance is about equal to the reference capacitance when the
microphone is not subject to an acoustic signal.
4. The microphone system of claim 1, wherein the primary diaphragm
defines a diaphragm plane when not subject to an acoustic signal,
and wherein the reference diaphragm is nominally within the
diaphragm plane, and is radially spaced from the primary
diaphragm.
5. The microphone system of claim 1, wherein the primary diaphragm
and the reference diaphragm define an electrical node.
6. The microphone system of claim 1, wherein the primary diaphragm,
the mechanical coupler, and the reference diaphragm define an
electrical node.
7. The microphone system of claim 1, further comprising a substrate
having a frontside and a backside, the substrate further comprising
a backside cavity extending into the backside of the substrate, the
primary diaphragm suspended from the frontside and exposed through
the backside cavity.
8. The microphone system of claim 1, the system further comprising
a differential circuit having a non-inverting input and an
inverting input, the primary capacitance electrically coupled to
the non-inverting input and the reference capacitance coupled to
the inverting input.
9. The microphone system of claim 8, wherein the primary diaphragm
is suspended parallel to the backplate, and the reference diaphragm
is suspended parallel to the reference electrode.
10. The microphone system of claim 9, wherein the primary
capacitance has a nominal primary capacitance value, and the
reference capacitance has a nominal reference capacitance value
equal to the primary capacitance value.
11. The microphone system of claim 1, further comprising a
substrate, the substrate comprising the backplate and the reference
diaphragm.
12. The microphone system of claim 11, the substrate further
comprising a trough opposite the reference diaphragm.
13. The microphone system of claim 1, wherein the primary diaphragm
defines a diaphragm plane, and the reference diaphragm has an
annular geometry and surrounds the primary diaphragm within the
diaphragm plane.
14. The microphone system of claim 13, wherein the reference
diaphragm and the primary diaphragm are concentric.
15. A packaged microphone system for detecting an acoustic signal,
the microphone system comprising: a housing comprising: a base; a
lid coupled to the base to form a cavity, one of the base and lid
forming an aperture extending from the cavity to the environment
outside of the housing; a MEMS microphone secured within the cavity
and being in acoustic communication with the aperture, the MEMS
microphone comprising: a substrate forming a backside cavity, the
substrate being coupled to the base such that the backside cavity
covers the aperture; a backplate supported by the substrate; a
primary diaphragm suspended from the substrate and forming a
variable primary capacitance with the backplate; a reference
electrode supported by the substrate; a reference diaphragm
suspended from the substrate and laterally spaced from the primary
diaphragm, the reference diaphragm forming a variable reference
capacitance with the reference electrode; a mechanical coupler
coupling the primary diaphragm to the reference diaphragm, the
reference diaphragm being configured to move in antiphase to the
primary diaphragm when the acoustic signal impinges on the primary
diaphragm.
16. The packaged microphone system of claim 15, wherein the
reference diaphragm is not directly exposed to the aperture such
that there is no direct acoustic path from the base aperture to the
reference diaphragm.
17. The packaged microphone system of claim 15, wherein the primary
diaphragm and the reference diaphragm are concentric.
18. A microphone system for detecting an acoustic signal, the
microphone system comprising: a micromachined device comprising a
backplate; a primary diaphragm suspended parallel to the backplate,
and spaced from the backplate by a variable primary gap to form a
variable primary capacitance across the primary gap, the primary
capacitance varying in response to the acoustic signal impinging on
the primary diaphragm; a reference electrode; a reference diaphragm
suspended parallel to the reference electrode and spaced from the
reference electrode by a variable reference gap to form a variable
reference capacitance with the reference electrode across the
variable reference gap; and means for mechanically coupling the
primary diaphragm to the reference diaphragm, the mechanically
coupling means being configured to vary the reference gap inversely
and proportionately to the primary gap in response to impingement
of the acoustic signal on the primary diaphragm such that the
reference capacitance varies inversely and proportionately to the
primary capacitance.
19. The microphone system of claim 18, wherein the means for
mechanically coupling comprises: means for supporting a torsion bar
from a substrate, the torsion bar between the primary diaphragm and
the reference diaphragm; and a beam coupled to the torsion bar, the
primary diaphragm, and the reference diaphragm, wherein the beam
mechanically couples the primary diaphragm to the reference
diaphragm.
20. The microphone system of claim 19, the system further
comprising a circuit for producing an output signal in response to
changes in the primary capacitance and the reference capacitance
Description
TECHNICAL FIELD
[0001] The present invention relates to microphones, and more
particularly to micro-electro-mechanical ("MEMS") microphones.
BACKGROUND ART
[0002] Micro-electromechanical ("MEMS") components and processes
are used for a wide variety of different devices. For example,
among other things, they are commonly used for producing
accelerometers to detect acceleration, pressure sensors to detect
pressure, power scavengers to accumulate power, and as microphones
to capture acoustic signals.
[0003] MEMS capacitive microphones in particular have found a wide
variety of different uses, such as in consumer electronics (e.g.,
cameras), smart phones and personal computers. This wide use is due
in part to their higher stability and smaller size than traditional
condenser microphones. As the technology improves, there is an
increasing demand to enhance acoustic quality--in particular,
higher sensitivity. In general, the dynamic range of a microphone
is limited at the upper end by total harmonic distortion and at the
lower end by its noise floor.
SUMMARY OF VARIOUS EMBODIMENTS
[0004] In a first embodiment of the invention, a microphone system
for detecting an acoustic signal includes a micromachined device
having a backplate; a primary diaphragm separated from the
backplate by a variable primary gap; the primary diaphragm and the
backplate forming a variable primary capacitance across the primary
gap, such that the primary capacitance varies in response to the
acoustic signal impinging on the primary diaphragm; a reference
electrode; a reference diaphragm separated from the reference
electrode by a variable reference gap, the reference diaphragm
forming a variable reference capacitance with the reference
electrode across the variable reference gap; and a mechanical
coupler coupling the primary diaphragm to the reference diaphragm.
The mechanical coupler is configured to vary the reference gap
inversely and proportionately to the variation of primary gap, and
to vary the reference capacitance inversely and proportionately to
the reference capacitance.
[0005] In some embodiments, the mechanical coupler includes a
torsion bar supported by at least one anchor coupled to a
substrate, the torsion bar between the primary diaphragm and the
reference diaphragm; and a beam coupled to the torsion bar, the
primary diaphragm, and the reference diaphragm, the beam
mechanically coupling the primary diaphragm to the reference
diaphragm.
[0006] In some embodiments, the primary capacitance is about equal
to the reference capacitance when the microphone is not subject to
an acoustic signal.
[0007] In some embodiments, the primary diaphragm defines a
diaphragm plane when not subject to an acoustic signal, and wherein
the reference diaphragm is nominally within the diaphragm plane,
and is radially spaced from the primary diaphragm.
[0008] In some embodiments, the primary diaphragm and the reference
diaphragm define an electrical node. Indeed, in some embodiments,
the primary diaphragm, the mechanical coupler, and the reference
diaphragm define an electrical node.
[0009] Some embodiments also include a substrate having a frontside
and a backside, and the substrate includes a backside cavity
extending into the backside of the substrate, and the primary
diaphragm suspended from the frontside and exposed through the
backside cavity.
[0010] In some embodiments, the system further includes a
differential circuit having a non-inverting input and an inverting
input, the primary capacitance electrically coupled to the
non-inverting input and the reference capacitance coupled to the
inverting input. Indeed, in some embodiments the differential
circuit is a differential amplifier. In some embodiments, the
primary diaphragm is suspended parallel to the backplate, and in
some embodiments the reference diaphragm is suspended parallel to
the reference electrode.
[0011] In some embodiments, the primary capacitance has a nominal
primary capacitance value, and the reference capacitance has a
nominal reference capacitance value equal to the primary
capacitance value. Further, in some embodiments the primary
capacitance has a nominal primary capacitance value, and the
reference capacitance has a nominal reference capacitance value,
and the reference capacitance has a nominal reference capacitance
value equal to the primary capacitance value.
[0012] In some embodiments, the microphone has a substrate that
includes the backplate and the reference diaphragm. Further, in
some embodiments the substrate includes a trough opposite the
reference diaphragm.
[0013] In some embodiments, the primary diaphragm defines a
diaphragm plane, and the reference diaphragm has an annular
geometry and surrounds the primary diaphragm within the diaphragm
plane. Indeed, in some embodiments the reference diaphragm and the
primary diaphragm are concentric.
[0014] In another embodiment, a packaged microphone system for
detecting an acoustic signal includes a housing with a base; a lid
coupled to the base and covering the aperture to form a cavity, one
of the base and the lid forming an aperture extending from the
cavity to the environment outside of the housing; and also includes
a MEMS microphone secured within the cavity and being in acoustic
communication with the aperture, the MEMS microphone forming a
backside cavity, and being coupled to the base such that the
backside cavity covers the aperture; a backplate supported by the
substrate, a primary diaphragm suspended from the substrate and
forming a variable primary capacitance with a backplate; a
reference diaphragm suspended from the substrate and laterally
spaced from the primary diaphragm, the reference diaphragm forming
a variable reference capacitance with the reference electrode; a
mechanical coupler coupling the primary diaphragm to the reference
diaphragm, the reference diaphragm being configured to move in
antiphase to the primary diaphragm when an acoustic signal impinges
on the primary diaphragm.
[0015] In some embodiments, the reference diaphragm is not directly
exposed to the aperture such that there is no direct acoustic path
from the base aperture to the reference diaphragm.
[0016] In some embodiments, the primary diaphragm and the reference
diaphragm are concentric.
[0017] In another embodiment, a microphone system for detecting an
acoustic signal includes a micromachined device having a backplate;
a primary diaphragm suspended parallel to the backplate and
separated from the backplate by a variable primary gap to form a
variable primary capacitance across the primary gap, the primary
capacitance varying in response to the acoustic signal impinging on
the primary diaphragm; a reference electrode; a reference diaphragm
suspended parallel to the reference electrode and separated from
the reference electrode by a variable reference gap to form a
variable reference capacitance with the reference electrode across
the variable reference gap; and means for mechanically coupling the
primary diaphragm to the reference diaphragm, the mechanically
coupling means being configured to vary the reference gap inversely
and proportionately to the primary gap in response to impingement
of the acoustic signal on the primary diaphragm such that the
reference capacitance varies inversely and proportionately to the
primary capacitance.
[0018] In some embodiments, the means for mechanically coupling
includes means for supporting a torsion bar from a substrate, the
torsion bar between the primary diaphragm and the reference
diaphragm; and a beam coupled to the torsion bar, the primary
diaphragm, and the reference diaphragm, wherein the beam
mechanically couples the primary diaphragm to the reference
diaphragm.
[0019] In some embodiments the system further includes a circuit
for producing an output signal in response to changes in the
primary capacitance and the reference capacitance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The foregoing features of embodiments will be more readily
understood by reference to the following detailed description,
taken with reference to the accompanying drawings, in which:
[0021] FIGS. 1A and 1B schematically illustrate a MEMS
microphone;
[0022] FIGS. 2A-2K schematically illustrate embodiments of a MEMS
microphone with mechanically-coupled diaphragms, and various
features of such microphones;
[0023] FIG. 3 schematically illustrates an embodiment of a MEMS
microphone;
[0024] FIG. 4A schematically illustrates inversely variable
capacitances of an embodiment of a MEMS microphone;
[0025] FIG. 4B schematically illustrates a circuit for producing an
output signal from inversely variable capacitances;
[0026] FIG. 5 is a flow chart that illustrates a method of
fabricating an embodiment of a substrate for a MEMS microphone;
[0027] FIGS. 6A-6E schematically illustrate features of an
embodiment of a substrate for a MEMS microphone at various stages
of fabrication;
[0028] FIG. 7A schematically illustrates an embodiment of a
packaged MEMS microphone system;
[0029] FIG. 7B schematically illustrates an alternate embodiment of
a packaged MEMS microphone system;
[0030] FIGS. 8A-8C schematically illustrate alternate embodiments
of two-diaphragm microphones;
[0031] FIG. 9 schematically illustrates an alternate embodiment of
a packaged MEMS microphone system;
[0032] FIG. 10 schematically illustrates an alternate embodiment of
a microphone with conductive electrodes.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0033] In various embodiments, a microphone system produces an
output signal with sensitivity greater than many known MEMS
microphones, while maintaining a comparable height profile--in some
embodiments, they have the same height profile. To that end,
exemplary embodiments have mechanically-coupled primary and
reference diaphragms that move inversely to one another (i.e., when
one moves up, the other moves down) to significantly increase the
signal. In fact, illustrative embodiments often produce more
electrical signal from an acoustic signal impinging on primary
diaphragm than a prior art microphone having a like sized single
diaphragm. Details of illustrative embodiments are discussed
below.
[0034] FIG. 1A schematically illustrates a top, perspective view of
a MEMS microphone chip 100. FIG. 1B schematically shows a
cross-sectional view of the same MEMS microphone chip 100 along
section A-A. These two figures are discussed to detail some
exemplary components of a MEMS microphone.
[0035] As shown in FIGS. 1A and 1B, the microphone chip 100 has the
chip base/substrate 101, one portion of which supports a backplate
102. The microphone 100 also includes a flexible diaphragm 103 that
is movable relative to the backplate 102. The diaphragm 103 is
suspended by springs 109, and the backplate 102 and diaphragm 103
are separated by a gap 108, and together form a variable capacitor
across gap 108. In some microphones, the backplate 102 is formed
from single crystal silicon (e.g., a part of a
silicon-on-insulator, or "SOI," wafer, which may be known as the
"device layer" 113), while the diaphragm 103 is formed from
deposited polysilicon. In other embodiments, however, the backplate
102 and diaphragm 103 may be formed from different materials.
[0036] In the embodiment shown in FIG. 1B, the microphone substrate
101 includes the backplate 102 and other structures, such as a
bottom wafer 111 and a buried oxide layer 112 of an SOI wafer. A
portion of the substrate 101 also forms a backside cavity 104
extending from the bottom 115 of the substrate 101 to the bottom of
the backplate 102. To facilitate operation, the backplate 102 has a
plurality of through-holes 107 that lead from gap 108 (i.e., a gap
between the diaphragm 103 and backplate 102) to the backside cavity
104. As such, the diaphragm 103 is exposed through the backside
cavity 104. One or more terminals 110 may electrically couple
features of the microphone, such as diaphragm 103 or backplate 102,
for example, to circuitry on the MEMS device, or external
circuitry.
[0037] It should be noted that various embodiments are sometimes
described herein using words of orientation such as "top,"
"bottom," or "side." These and similar terms are merely employed
for convenience and typically refer to the perspective of the
drawings. For example, the substrate 101 is below the diaphragm 103
from the perspective of FIGS. 1A and 1B. However, the substrate 101
may be in some other orientation relative to the diaphragm 103
depending on the orientation of the MEMS microphone 100. Thus, in
the present discussion, perspective is based on the orientation of
the drawings of the MEMS microphone 100.
[0038] In operation, acoustic signals strike the diaphragm 103,
causing it to vibrate, thus varying the gap 108 between the
diaphragm 103 and the backplate 102 to produce a changing
capacitance. The diaphragm may generally move in a plunger-like
motion, in which the diaphragm 103 remains parallel to the
backplate 102 as it moves towards, or recedes from, the backplate
102.
[0039] Unlike the microphone 100 described above shown in FIGS. 1A
and 1B, illustrative embodiments have at least two diaphragms.
Specifically, as shown in FIG. 2A and others, such a microphone
system 200 includes two diaphragms: a first diaphragm 103 (which
may be known as the "primary" diaphragm) and a second diaphragm 203
(which may be known as the "reference" diaphragm), both of which
are suspended above the substrate 101. In some embodiments, the
primary diaphragm 103 is acoustically open to the air and acoustic
energy traveling from the backside 115 of the substrate 101 through
the cavity 104 and the backplate 102, while the reference diaphragm
203 is isolated from the air and acoustic energy traveling from the
backside 115 of the substrate 101 by layers of the substrate 101
under it.
[0040] When the microphone 200 is not subject to an impinging
acoustic signal, the diaphragms 103 and 203 are nominally in the
same plane (the plane, which may be known as the "nominal plane" or
the "diaphragm plane," is schematically illustrated in profile by
line 201). As schematically illustrated in FIG. 2A, microphone 200
is thinner than a dual-backplate microphone which would have a
second backplate above the diaphragm, such that the diaphragm 103
would be sandwiched between backplate 102 and the second
backplate.
[0041] As with the microphone 100 shown in FIGS. 1A and 1B, the
primary diaphragm 103 forms a variable capacitance (which may be
known as the "variable primary capacitance") with the backplate (or
"backplate electrode") 102 across the gap 108. To this end, the
backplate 102 may include doped polysilicon, for example, although
in other embodiments the backplate 102 may include a conductive
member (e.g., as schematically illustrated in FIG. 10).
[0042] When the microphone 200 is not subject to an impinging
acoustic signal, the primary diaphragm 103 may be said to form a
"nominal" primary capacitance with the backplate 102. That
capacitance is a function, in part, of the surface area of the
primary diaphragm 103, the surface area of the backplate 102, and
the distance or gap 108 between them, according to the capacitance
equation: C=(.epsilon.r)(.epsilon.o)(A/d), where "C" is the
capacitance, ".epsilon.r" is the relative static permittivity,
".epsilon.o" is the electric constant, "A" is the surface area of
the overlap of the areas of the primary diaphragm and the
backplate, and "d" is the distance by which the primary diaphragm
103 and the backplate 102 are separated. Among other things, this
equation shows that the capacitance between the primary diaphragm
103 and the backplate 102 increases linearly as the distance
between them decreases, if all other factors remain the same.
[0043] In a similar manner, the reference diaphragm 203 forms a
variable reference capacitance with a reference electrode 202, in
or on the substrate 101, across a gap 225. To this end, the
reference electrode 202 may include doped polysilicon, for example,
although in other embodiments the reference electrode 202 may
include a conductive member (e.g., as schematically illustrated in
FIG. 10).
[0044] The reference electrode 202 is electrically isolated from
the rest of the substrate 101 by insulative buffer vias 206. In
some embodiments, the buffer vias 206 may be oxide, for example,
and may be contiguous with the buried oxide layer 112. In some
embodiments, the buffer vias 206 have an annular shape, such that
the reference electrode 202 surrounds one buffer via 206 and
another buffer via 206 surrounds the reference electrode 202. In
other embodiments, each of the vias 206 may be just an air-filled
trench.
[0045] When the microphone 200 is not subject to an impinging
acoustic signal, the reference diaphragm 203 may be said to form a
"nominal reference capacitance" with the reference electrode 202.
According to the capacitance equation above, the reference
capacitance is a function, in part, of the gap 225 between the
reference electrode 202 and the reference diaphragm 203.
[0046] In some embodiments, the surface area of the reference
diaphragm 203 is equal to the surface area of the primary diaphragm
103, although in other embodiments the surface areas may be
different. Similarly, in some embodiments the nominal reference
capacitance is equal to the nominal primary capacitance, although
in other embodiments to the capacitances may be difference.
[0047] One embodiment of the mechanically coupled primary diaphragm
103 and reference diaphragm 203 is schematically illustrated in
FIG. 2D. In this embodiment, the primary diaphragm 103 is circular,
and the reference diaphragm 203 is annular (i.e., has the shape of
an annulus/toroid, in which the diaphragm 203 occupies the space
between two concentric circles, and the inner circle defines a
reference diaphragm aperture 240). In some embodiments in which an
acoustic signal impinges on the primary diaphragm 103, the
diaphragm aperture (e.g., 240) may serve to reduce or eliminate the
amount of acoustic energy that would otherwise impinge on the
reference diaphragm 203, for example by spacing the reference
diaphragm 203 laterally from the primary diaphragm 103. In some
embodiments, the primary diaphragm 103 is concentric with and in
the same plane as, and yet laterally spaced from, the reference
diaphragm 203. In some embodiments, the reference diaphragm 203 is
a continuous ring, as schematically illustrated in FIG. 2D for
example, but in other embodiments the reference diaphragm 203 may
be segmented. The diaphragms 103 and 203 are not limited to
circular or annular shapes, however. In other embodiments, one or
both of the diaphragms may be non-circular, including square or
rectangular shaped diaphragms, for example.
[0048] In some embodiments, the backplate 102 and reference
electrode 202 are electrically independent of one another, such
that the primary capacitance is separate from the reference
capacitance. In some embodiments, the primary diaphragm 103 and the
reference diaphragm 203 may be an electrical node, while the
primary capacitance is separate from the reference capacitance.
Indeed, in other embodiments, the primary diaphragm 103 and the
reference diaphragm 203 and the mechanical coupler 210 may be an
electrical node.
[0049] In illustrative embodiments, the primary diaphragm 103 and
the reference diaphragm 203 may be electrically separate from one
another, such that the primary capacitance is separate from the
reference capacitance. Indeed, in some embodiments, the backplate
102 and reference electrode 202 may be an electrical node, while
the primary capacitance is separate from the reference capacitance.
However, the primary capacitance and reference capacitance may
still have a relationship to one another, as described below.
[0050] When an acoustic signal impinges on the primary diaphragm
103, acoustic energy in the signal causes the primary diaphragm 103
to move, as described in connection with microphone 100. For
example, in some embodiments, the primary diaphragm 103 remains
parallel to the backplate 102 as it moves in response to acoustic
energy.
[0051] However, that same acoustic energy--that is, the acoustic
energy impinging on the primary diaphragm 103--also causes the
motion of the reference diaphragm 203 relative to the substrate
101, or more particularly, relative to the reference electrode 202.
Indeed, in some embodiments, the acoustic signal does not directly
impinge on the reference diaphragm 203, so that the acoustic energy
in the acoustic signal does not directly cause movement of the
reference diaphragm 203. In some embodiments, acoustic energy from
an acoustic signal impinging on one side of a primary diaphragm 103
may leak under the primary diaphragm 103, or around to the other
side of the primary diaphragm 103, and thereby impinge on one side
of the reference diaphragm 203. However, in some embodiments the
effect of any such leakage is likely to be negligible.
[0052] The primary diaphragm 103 is mechanically coupled to the
reference diaphragm 203 by one or more mechanical couplers 210. In
illustrative embodiments, each mechanical coupler 210 includes a
beam 211 suspended above the substrate 101, and coupled at one end
212 to the primary diaphragm 103, and at the other end 213 to the
reference diaphragm 203. The beam 211 preferably will not bend
along its length 215. However, in some embodiments the beam 211 may
extend slightly along its length 215 (FIG. 2G) as the diaphragms
103, 203 move. In some embodiments, the beam 211 may be in the same
layer as, and be made of the same material as, the primary
diaphragm 103 and the reference diaphragm 203.
[0053] In some embodiments, the beam 211 is supported from the
substrate 101. In some embodiments, the beam is supported at a
point 216 between its two ends 212, 213, and in some embodiments
may be support at the center point of its length 215.
[0054] The supporting structure 210 is coupled to the substrate 101
and is coupled to the beam 211 in such a way as to allow the beam
211 to rotate along an axis 220 normal to its length 215 and
parallel to the plane 101. In FIG. 2A, the plane of the substrate
is schematically illustrated in profile by line 230. As such, the
beam 211 may be described as moving or pivoting similar to a
teeter-totter (or seesaw), as schematically illustrated in FIG. 2H,
for example.
[0055] One embodiment of a mechanical coupler 210 is schematically
illustrated in FIGS. 2E-2H, in which FIG. 2E schematically
illustrates a plan view of mechanical coupler 210, FIG. 2F
schematically illustrates a side view of mechanical coupler 210
along cross-section B-B, and FIGS. 2G and 2H schematically
illustrate side views of mechanical coupler 210 along cross-section
C-C (but also including anchor 218).
[0056] Mechanical coupler 210 includes two anchors 217, 218 coupled
to the substrate 101. Anchors 217 and 218 support a torsion bar 214
above the substrate 101. The torsion bar 214 is flexible, and may
twist along an axis 220 parallel to the substrate 101. For example,
if one end 212 of beam 211 moves downwards toward the substrate
101, torsion bar 214 twists so that the other end 213 of the
torsion bar moves upwards away from substrate 101, as schematically
illustrated in FIG. 2H, for example.
[0057] An alternate embodiment of a mechanical coupler is
schematically illustrated in FIG. 2I, and includes several beams
211 supported by a single, circular member 219. The circular ring
member 219 may be in the same layer as the beams 211, and thus be
supported above the substrate by a number of anchors (such as 217,
218), or may be supported by a single continuous anchor (e.g., 217)
between the ring 219 and the substrate 101.
[0058] Although various embodiments are schematically illustrated
has having four beams 211, other embodiments may have more or fewer
beams, and one or more separate mechanical couplers. For example,
some embodiments may have 2, 3 or even more mechanical couplers 210
placed between the primary diaphragm 103 and the reference
diaphragm 203. In some embodiments with multiple mechanical
couplers 210, the mechanical couplers 210 may be spaced evening
around the primary diaphragm 103, as schematically illustrated in
FIG. 2D for example, or spaced unevenly.
[0059] Alternate diaphragm embodiments are schematically
illustrated in FIGS. 2J and 2K. In FIG. 2J, the primary diaphragm
103 has a square or rectangular geometry, and reference diaphragm
203 has a matching shape with an aperture 228 to accommodate the
primary diaphragm 103, and a square or rectangular mechanical
coupler 229 with several beams 211. Of course, one or more
individual mechanical couplers 210 could be used in place of the
single square mechanical coupler 229.
[0060] FIG. 2K schematically illustrates another embodiment, in
which a primary diaphragm 103 is flanked by a two-part reference
diaphragm 253. The primary diaphragm 103 is coupled to each segment
253A part of the reference diaphragm 253 by mechanical couplers
210. The microphone 250 also has a backplate (e.g., 102) beneath
the diaphragm 103 to form a primary capacitance (e.g., 410), and a
reference electrode (e.g., 202) in or on the substrate 101 and
parallel to the reference diaphragm 253, so as to form a single
variable reference capacitance (e.g., 420). In alternate
embodiments, the primary diaphragm 103 may be flanked by only a
single reference-diaphragm segment 253A adjacent to but not
surrounding the primary diaphragm 103 (e.g., only one part 253A of
the two-part reference diaphragm 253 in FIG. 2K). In yet other
embodiments, the primary diaphragm 103 may be flanked by three or
more reference diaphragm segments 253A, such as the reference
diaphragm segments 253A in FIG. 2K for example.
[0061] In operation, when the primary diaphragm 103 moves in
response to an impinging acoustic signal, the reference diaphragm
203 moves an equal amount, but in the opposite direction. For
example, when the primary diaphragm 103 moves towards the substrate
101, the (proximal) end 212 of the beam 211 that is coupled to the
primary diaphragm 103 also moves towards the substrate 101, as
schematically illustrated in FIG. 2B. However, the other (distal)
end of the beam 213 moves in the opposite direction--that is, away
from the substrate 101. The distal end 213 of the beam 211 is
connected to the reference diaphragm 203, and as such, the motion
of the beam 211 moves or displaces the reference diaphragm 203, as
schematically illustrated in FIG. 2B.
[0062] In some embodiments, the displacement of the primary
diaphragm 103 by a given amount causes in equal displacement of the
reference diaphragm in the opposite direction. For example, if an
impinging acoustic signal causes the primary diaphragm 103 to move
a distance X towards the backplate 102, then the reference
diaphragm 203 moves a distance X away from the reference electrode
202, as schematically illustrated in FIG. 2B. Similarly, if an
impinging acoustic signal causes the primary diaphragm 103 to move
a distance X away from the backplate 102, then the reference
diaphragm 203 moves an equal distance X towards the reference
electrode 202, as schematically illustrated in FIG. 2C. As such,
the primary diaphragm 103 and reference diaphragm 203 may be
described as moving in antiphase, and in some embodiments, the
motion of the primary diaphragm 103 and reference diaphragm move
inversely and proportionately to one another. In some embodiments,
the dimensions and construction of the diaphragms 103, 203,
backplate 102 and the reference electrode 202 are configured such
that the primary capacitance (e.g., 410) and reference capacitance
(e.g., 420) vary inversely and proportionately to one another
(e.g., if the primary capacitance 410 doubles, then the reference
capacitance 420 is reduced by half).
[0063] In some embodiments, the primary diaphragm 103 moves such
that it remains substantially parallel to the backplate 102, and
the reference diaphragm 203 moves such that it remains
substantially parallel to the reference electrode 202.
[0064] Similarly, when the primary diaphragm 103 moves away from
the substrate 101, the end 212 of the beam 211 that is coupled to
the primary diaphragm 103 also moves away from the substrate 101,
as shown in FIG. 2C. However, the other (distal) end 213 of the
beam 211 moves in the opposite direction--that is, towards the
substrate 101. The distal end 213 of the beam 211 is connected to
the reference diaphragm 203, and as such, the motion of the beam
211 moves the reference diaphragm 203 towards the substrate 101. As
described above, in some embodiments, the reference diaphragm 103
moves such that it remains substantially parallel to the backplate
102, and the reference diaphragm 203 moves such that it remains
substantially parallel to the reference electrode 202.
[0065] The variable primary capacitance 410 and variable reference
capacitance 420, which are schematically illustrated in FIG. 4A,
vary in response to the acoustic signal impinging on the primary
diaphragm 103. The varying capacitances 410, 420 may be processed
to produce an output signal representing the acoustic signal. For
example, some embodiments include a circuit for producing an output
signal in response to inverse changes in the primary capacitance
410 and the reference capacitance 420.
[0066] FIG. 4B schematically shows one embodiment of a circuit 400
for producing an output signal in response to inverse changes in
the primary capacitance and the reference capacitance. In circuit
400, the primary diaphragm 103 and the reference diaphragm 203 of a
multiple-diaphragm microphone, such as microphone 200 or 300 for
example, form an electrical node 430, and are supplied by a common,
D.C. bias voltage 431 ("Vbias"). The backplate 103 forms an
electrical node 401 with the non-inverting input 451 of
differential signal processing circuit 400, for example a
differential amplifier 450, and resistor 441. Similarly, the
reference electrode 202 forms an electrical node with the inverting
input 452 of differential signal processing circuit 450 and
resistor 442.
[0067] Because the voltage across each of the capacitances 410 and
420 is constant, the varying capacitance in the capacitances 410,
420 creates current flow into and out of the capacitances as those
capacitances 410, 420 vary in response to the acoustic signal. More
specifically, the current flow into a varying capacitor is
described by the following equation: i=V dC/dt, where "i" is the
current flow, "V" is the constant bias voltage, and "dC/dt" is the
time variation of the capacitor.
[0068] Current flowing from the variable capacitance 410 through
the resistor 441 produces a voltage at that node 401, which voltage
is coupled to the non-inverting input 451 of the buffer 450.
Similarly, current flowing from variable capacitance 420 and
through resistor 442 produces a voltage at the node 402, which
voltage is coupled to the inverting input 452 of the differential
signal processing circuit 450. Together, the voltages at the nodes
401 and 402 form a differential input to the differential signal
processing circuit 450. The differential signal processing circuit
450 buffers or amplifies that differential voltage input to produce
a differential voltage output ("Vo"=V+out-V-out) at terminals 453.
Of course, various embodiments of differential signal processing
circuits may produce a single-ended output signals.
[0069] In some embodiments, the nominal primary capacitance (410)
is equal to the nominal reference capacitance (420). As the
capacitances 410, 420 change in response to an impinging acoustic
signal, the output voltage signal (Vo) of the circuit 400 as a
function of time may be designated as "Vo(t)." Similarly, "dCp" is
the change of the primary capacitance (e.g., 410), "dCr" is the
change of the reference capacitance (e.g., 420), "Cpn" is the
nominal primary capacitance, and "Cm" is the nominal reference
capacitance.
[0070] In some embodiments, the nominal primary capacitance (Cpn),
nominal reference capacitance (Cm), and Vbias are constant, and
therefore do not vary with time. In other embodiments, the nominal
primary capacitance and nominal reference capacitance may be
unequal, or the change in capacitances 410, 420 in response to an
impinging acoustic signal (i.e., dCp and dCr) may not be equal. In
illustrative embodiments, the output signal (Vo) is proportional to
dCp-dCr.
[0071] An alternate embodiment of a microphone 300 is schematically
illustrated in FIG. 3. The embodiment 300 of FIG. 3 is similar to
the microphone 200 of FIG. 2A, but also includes one or more
damping relief chambers 301. Some MEMS devices experience a
phenomenon known as "squeeze film damping." The phenomenon arises
when air or other gas is compressed between a moving MEMS structure
and another surface. For example, in some MEMS microphone, squeeze
film damping may arise between a moving diaphragm and a
substrate.
[0072] In the microphone 200, squeeze film damping is not likely to
arise between the primary diaphragm 103 and backplate 102 because
any rising pressure in the gas between them is likely to be
relieved via through holes 107. However, there are no such holes in
the substrate 101 beneath the reference diaphragm 203.
[0073] In microphone 300, however, damping relief chambers (or
"trench") 301 beneath the reference diaphragm 203 provides a volume
into which such increasing pressure may be alleviated. In some
embodiments, damping relief chambers 301 may be closed-ended; in
other words, the chamber 301 is not a throughhole or aperture all
the way through the substrate 101 such that an acoustic signal
could pass through the substrate and reach the reference diaphragm
203 via the chamber 301. In some embodiments, however, a damping
relief chamber 301 may be an aperture passing all the way through
the substrate 101, but preferably such an aperture is not exposed
to the incoming acoustic signal in such a way as the acoustic
signal could pass through the substrate and reach the reference
diaphragm 203 via the chamber 301.
[0074] In microphone 300, the damping relief chambers 301 have an
opening 310 opposite the reference diaphragm 203. In some
embodiments, the opening 310 may be through the reference electrode
202, but in other embodiments may be laterally spaced from the
reference electrode 202.
[0075] The volume of a damping relief chamber 301 should be
sufficient to accommodate an inflow of gas as the reference
diaphragm 203 moves towards the substrate 101. To that end, the
depth 302 of an exemplary damping relief chamber 301 may range from
5 micrometers (5 um) to 500 um, and the width 303 may range between
10 um and 1000 um. In the embodiments of FIG. 3, the depth 302 of
the damping relief cavity 301 is less than the thickness of the
substrate 101, so that the damping relief cavity 301 is not open to
the environment on the side (101B) of the substrate 101 that is
opposite to the side (101T) at which the reference diaphragm 203 is
located. In some embodiments, as schematically illustrated in FIG.
3, one or more damping relief chambers extend downward from the
surface (101T) of the substrate 101 to a depth 302 that is below
the buried oxide layer 112 and into the bottom wafer 111 of an SOI
wafer. In some embodiments, as schematically illustrated in FIG. 3,
one or more damping relief chambers extend downward from the
surface (101T) of the substrate 101 to a depth 302 that is at least
half the thickness of the substrate 101.
[0076] In some embodiments, each a damping relief chamber 301 may
be a separate cavity. In other words, the damping relief chambers
301 may not be connected to one another. In other embodiments,
however, one or more damping relief chambers 301 may be
acoustically connected (other than by being exposed to the same
environments beneath the reference diaphragm 203). Some embodiments
include a single damping relief chamber 301, for example a single
chamber may surround the backplate 102. Such a damping relief
chamber 301 may have an annular geometry (i.e., a cross-section of
the damping relief chamber 301 forms an annulus in a plane within
the substrate, which plane is parallel to the substrate plane) such
that it follows an annular reference diaphragm 203.
[0077] FIG. 5 is a flow chart illustrating a process 500 of
fabricating a MEMS microphone, such as microphone 300 for example,
and FIGS. 6A-6E schematically illustrate a substrate of microphone
300 at various stages of fabrication.
[0078] The process includes providing a bottom wafer 111 (FIG. 6A)
at step 501. Next, one or more trenches 601 are etched into a top
surface 151 of the wafer 111, at step 502 (FIG. 6B), according to
etching processes known in the art of MEMS fabrication. For
example, a mask layer (not shown) may be deposited on surface 151
of wafer 111 everywhere except where the trench 601 is to be
etched. An etching material may then be applied to the top surface
151 of the wafer 111, so as to etch the trench 601 into the wafer
111. The mask layer may then be removed.
[0079] Next, a top wafer (or device layer wafer) 113 is provided at
step 503 (FIG. 6C). The top wafer 113 includes an insulator layer
112, such as an oxide layer for example. If the top wafer does not
have an insulator layer 112, then an insulator layer 112 may be
deposited on one side 150 of the top wafer 113.
[0080] The top wafer 113 is then bonded to the bottom wafer 111 at
step 504 (FIG. 6D), such that the insulator layer 112 is sandwiched
between the top wafer 113 and bottom wafer 111. As such, the bonded
wafers 111, 113 and insulator layer 112 form, in essence, an SOI
wafer. If desired, the top wafer 113 may optionally be thinned at
step 505 (FIG. 6E). Finally, the trench 601 is opened or exposed by
etching an opening 310 through the top wafer 113 and insulator
layer 112 at step 506, to form the dampening relief chamber 301.
The opening 310 may, for example, be formed during the formation of
throughholes 107.
[0081] The remaining microphone structures (such as the backplate,
reference electrode, primary diaphragm, reference diaphragm,
mechanical coupler 210, and connection terminals 110, for example)
may be fabricated by processes known in the art. For example, a
process for fabricating a MEMS microphone on an SOI wafer is
described in U.S. patent application publication number
2009/0202089, the content of which is incorporated herein, in its
entirety, by reference. The structure fabricated in that published
application does not include a reference diaphragm or a reference
electrode, but a reference diaphragm could be fabricated in the
same way, from the same materials and layers, and at the same time
as that microphone's diaphragm, and a reference electrode may be
fabricated using processes known in the art. Such a process would
also include fabricating mechanical coupler 210, and could do so in
the same way, using the same layers of material, and at the same
time as forming that microphone's diaphragm. For example, such a
process could form anchors 217, 218 from the insulator layer (212)
and form a beam 211 and torsion bar 214 from the same layer of
material from which the diaphragm and reference diaphragm or
diaphragms are formed.
[0082] In another embodiment of the system 700, a MEMS microphone,
such as microphones 200 or 300 for example, is secured within a
housing 701 having an aperture 704 for receiving acoustic signals.
To that end, the housing 701 includes a base 702 and a lid/cover
703 coupled to the base 702. The lid 703 and the base 702 together
form a chamber 710 containing the microphone 200. In some
embodiments, the lid 703 may be hermetically sealed to the base 702
so that the only acoustic path into the chamber 710 is via an
aperture 704, which can extend through the base 702 (e.g., FIG. 7A)
or lid 703 (e.g., FIG. 7B). More specifically, in some embodiments,
the base 702 includes a base aperture 704 to allow sound (e.g., an
acoustic signal) to enter the chamber 710 from a source outside of
the housing 701 and impinge on the primary diaphragm 103 of the
microphone 200.
[0083] Items other than the microphone 200, such as an ASIC 720,
may also occupy the chamber 710. The microphone 200 may be
electrically coupled to the base 702 in ways known in the art, such
as through wirebonds 709 or solder bumps between the microphone 200
and base 702, to name but a few examples.
[0084] In the embodiment of FIG. 7A, the microphone 200 is
physically coupled to the base 702 such that the backside cavity
104 straddles the aperture base 704. More specifically, the base
aperture 704 is aligned with the backside cavity 104 so that an
incoming acoustic signal may pass through the aperture 704, enter
the backside cavity 104, and ultimately impinge on the primary
diaphragm 103.
[0085] In some embodiments, a microphone system 700 may be coupled
to an underlying substrate 770, such as, a printed circuit board or
the housing of a larger assembly (e.g., the body of a cell phone or
hearing aid). The substrate 770 may include a corresponding
substrate aperture 771 aligned with base aperture 704, to allow an
acoustic signal to enter the housing 701 from a source on the
opposite side of the substrate 770.
[0086] An alternate embodiment of a packaged microphone system 780
is schematically illustrated in FIG. 7B. In system 780, the lid
703--not the base 702--includes an aperture 781. Microphone 200 is
physically coupled to the lid 703 and straddles aperture 781 such
that aperture 781 is aligned with the backside cavity 104.
[0087] In the embodiments illustrated and described above, the
center diaphragm has been the primary diaphragm (e.g., 103), and
the surrounding diaphragm has been the reference diaphragm (e.g.,
203). In other embodiments, however, those roles may be reversed.
For example, in one embodiment 800 schematically illustrated in
FIG. 8A, shows three layers (811, 812, and 813) of
silicon-on-insulator substrate 801. The outer diaphragm (e.g., 803)
receives the incoming acoustic signal, and the mechanical couplers
210 transmit the resulting motion to the inner diaphragm 823. As
such, the outer diaphragm 803 is the "primary" diaphragm, and the
inner diaphragm 823 is the "reference" diaphragm. Indeed, in some
embodiments, a primary diaphragm 803 may be annular, and may
surround and be concentric with the reference diaphragm 804.
[0088] As shown in FIG. 8A, the backside cavity 804 and backplate
802 are below the primary diaphragm 803, and surround a damping
relief cavity 830, which is beneath reference diaphragm 823. One or
more openings 310 lead from the damping relief cavity 830 to the
gap 850 between the substrate 801 and the reference diaphragm 823.
Buffer vias 206 electrically isolate the backplate 802 from the
remainder of layer 113.
[0089] A cross-section of microphone 800 along line F-F is
schematically illustrated in FIG. 8B. The cross-section of FIG. 8B
is taken through the top layer 813, and so does not show the
diaphragms 803, 823, or the mechanical couplers 210.
[0090] In this illustrative embodiment, the backplate 802 includes
two concentric circular backplate portions or electrodes 802A.
Backplate portions 802A are supported by, and electrically coupled
to each other by, supporting members 805. The reference electrode
824 and backplate portions 802A are supported by insulative bridges
825. One or more of insulative bridges 825 may also include or
carry a conductor 826 to electrically couple reference electrode
824 to an electrical terminal 110R or circuit.
[0091] An alternate embodiment 860 is schematically illustrated in
FIG. 8C. In that embodiment, the backplate 802 includes several
radial electrode segments 802B. Electrode segments 802B are
electrically coupled to each other by conductive arc 802C. An
insulator ring 207 electrically isolates the reference electrode
824. A connector 826 electrically couples the reference electrode
824 to a terminal 110R.
[0092] Another embodiment of a packaged microphone system 900 is
schematically illustrated in FIG. 9, and includes a microphone 800
in which the outer diaphragm 803 is the primary diaphragm (e.g.
410), and the inner diaphragm 823 is the reference diaphragm (e.g.,
420). In system 900, the substrate 702 includes an aperture 704.
Microphone 800 straddles the aperture 704 such that the backside
cavity 804 is aligned with the aperture 704. A portion of the
substrate 101 remains between the aperture 704 and the reference
diaphragm 823, and mitigates the transmission of acoustic energy
through aperture 704 to the reference diaphragm 823. In this
embodiment, there is no direct acoustic path from the aperture 704
to reference diaphragm 823.
[0093] Although illustrative embodiments described above show a
backplate (e.g., 102) and reference electrode (e.g., 202) as doped
regions of a substrate (e.g., 101), any of the embodiments above
may alternately have a conductive material on the substrate (e.g.,
101) to form the backplate (e.g., 102) or reference electrode
(e.g., 202). For example, FIG. 10 schematically illustrates an
alternate embodiment of microphone 200, in which the backplate 102
includes a conductive electrode 102A supported by the substrate
101, and the reference electrode 202 includes a conductive
reference electrode 202A supported by the substrate 101. In some
embodiments, the substrate 101, or top layer 113 of the substrate
101, may be non-conductive. Indeed, in some embodiments, buffer
vias 206 may be omitted.
[0094] Definitions. As used in this description and the
accompanying claims, the following terms shall have the meanings
indicated, unless the context otherwise requires:
[0095] The term "aligned" used in reference to two apertures (or in
reference to an aperture and a backslide cavity), means that the
plan profiles of the apertures (or the aperture and the backside
cavity) overlap such that a linearly directed object could pass
through both apertures. Two aligned apertures are schematically
illustrated in FIGS. 7A-7B, for example. A linearly directed object
(double-headed arrow 750) can pass through the two apertures. The
term "aligned" used in reference to an aperture (for example 704)
and a backslide cavity (for example, 104), means that the aperture
(704) and backside cavity (104) are arranged such that a linearly
aligned object (750) could pass through the aperture (704) and into
the backside cavity (104).
[0096] The term "direct acoustic path" means an acoustic path by
which an acoustic signal traveling in a straight line (e.g., dashed
line 760 in FIG. 7) may pass through an aperture and impinge on a
diaphragm without passing through an impeding surface. In a
microphone (e.g., 200) with a backplate (e.g., 102), an acoustic
signal has a direct acoustic path to a diaphragm (e.g., 103) if the
acoustic signal may pass through holes (e.g., 107) from the
backside cavity (e.g., 104) to the diaphragm. In some embodiments,
the structure and dimensions of a MEMS microphone prohibit a direct
acoustic path through the backside cavity to some other feature;
for example, an acoustic signal traveling in a straight line cannot
propagate through the backside cavity and impinge on the reference
diaphragm. In some embodiments, a microphone (e.g., 200) is
deployed or installed (e.g., as in a housing 701) such that there
is no direct acoustic path from to the reference diaphragm 203. In
other words, in such embodiments, if any acoustic energy from an
acoustic signal reaches reference diaphragm 203, it may do so only
indirectly by, for example, taking a non-straight path between the
primary diaphragm 103 and substrate 101, or by passing between the
primary diaphragm 103 and mechanical coupler 210 and bending, or
reflecting (for example within a chamber 710) to reach the
reference diaphragm 203. However, eliminating a direct acoustic
path to a reference diaphragm 203 is not a limitation of all
embodiments.
[0097] The embodiments described above are intended to be merely
exemplary; numerous variations and modifications will be apparent
to those skilled in the art. All such variations and modifications
are intended to be within the scope of the present invention as
defined in any appended claims.
* * * * *